Critical Technical Issues for SRF’s Advance

The planned and potential applications of SRF technology fall into four broad areas, each with a set of critical technical issues that need to be addressed by R&D.

Low- to medium-current CW accelerators

Examples of this category include the CEBAF 12 GeV upgrade—that is, the imminent doubling of the energy of CEBAF, the large Jefferson Lab accelerator that serves nuclear physics—as well as the Rare-Isotope Accelerator now being planned by U.S. nuclear astrophysicists and others. Because of the CW operation, the cavities in these applications will operate at relatively modest gradient for optimum overall efficiency. RF losses (and by implication, refrigeration capacity) will, on the other hand, be a major cost driver. Beam currents being low, the required installed RF power will be dictated by the level of microphonics (the coupling of mechanical vibrations to the RF fields), not by the beam power.

The critical technical issues in this category are:

• Cavities with low losses and high Q 0 in CW operation at relatively modest gradient (~20–25 MV/m)
• New cavity geometries
• Control and mitigation of microphonics; RF control of cavities
• Consistency in the performance of cavities
• Low-heat-leak cryostat designs

Pulsed high-current proton or ion accelerators

Examples in this category include research facilities like Oak Ridge National Laboratory’s Spallation Neutron Source (SNS), for which Jefferson Lab built an SRF proton accelerator and an associated cryogenic system. Because of the high beam current, the gradients will be modest (10–20 MV/m) and will be dictated by the capabilities of the RF power sources and power couplers. The pulsed operation could generate large frequency excursions that will need to be mitigated.

The critical issues are:

• Dynamic behavior of SRF cavities; frequency control
• Beam halo; activation
• New cavity geometries
• HOM (higher-order mode) power generation and extraction
• RF power couplers